The purpose of this project is to understand better the development of the endometrium and oocyte in women and to investigate the role of gonadal steroids in these processes. Efforts during this reporting period characterized the biologic action of the progesterone receptor modulator CDB2914, showing it to inhibit endometrial development when given to women in the luteal phase. Fifty regularly cycling women were studied during control, treatment, and post-treatment cycles. Each received a single dose of CDB2914 given after ovulation and within two days of the LH surge. Four to six days after treatment, a transvaginal ultrasound was done to measure endometrial thickness and ovarian cysts; an endometrial biopsy was performed for morphologic examination and immunohistochemical evaluation of estrogen and progesterone receptors; blood was obtained for chemistries, liver function tests, complete blood count, estradiol and progesterone levels. CDB2914 caused a significant dose-dependent decrease in endometrial thickness, a delay in endometrial maturation as judged by Noyes criteria, and an increase in glandular progesterone receptors among the treatment groups. While there was no significant increase in glandular estrogen receptors after CDB 2914, there was a greater difference between estrogen receptors in glands versus stroma. CDB-2914 did not alter circulating estradiol and progesterone levels or the menstrual cycle timing. No adverse effects were observed. This suggests that CDB 2914 has a long-lasting effect to inhibit differentiation of the endometrium. The recent discovery of a second ER estrogen receptor, beta, and the oncoprotein Brx, which augments estogen activity, suggest that estrogen action is regulated in a more complex way than previously appreciated. A second study during this year was designed to characterize the spatial and temporal localization of ER alpha and beta and the Brx in the endometrium of normally cycling women throughout the menstrual cycle. Endometrial biopsy samples later judged as in-phase, were obtained from 43 normally cycling women during the proliferative and secretory phases of the menstrual cycle. Histological dating was performed using the criteria of Noyes. Immunohistochemical staining was carried out using antibodies directed against ER alpha, ER beta and Brx. ERa had a nuclear localization that was greatest in the epithelium during the proliferative phase and decreased during the late secretory phase. ER beta showed a similar nuclear and epithelial cell preference but the intensity of ER beta immunostaining was increased during the secretory phase. Brx was uniformly localized within the cytoplasm of glandular and luminal epithelial during the proliferative phase and became localized to both the nucleus and cytoplasm during the secretory phase. These different patterns of staining during the menstrual cycle with nuclear down regulation of ER alpha and upregulation of ER beta in the glandular epithelium during the secretory phase suggest differential regulation, and possibly roles, of the two receptors in the endometrium.